I've travelled around Portobello, Alfie's, Gray's, Camden Passage, Bermondsey, Covent Garden & other places looking for Heuers, and pickings are usually slim. One dealer at Gray's (Bond St tube) usually has a couple in stock, but their prices aren't great; my old Heuer automatic started dying around five years ago, and I spent two or three years looking for a replacement before I found this board.
However, one place you must go, is Clerkenwell (Farringdon tube), where there is still a scattering of stores. Clerkenwell Road was traditionally where lots of watch dealers were based, many are gone, but it's still a fascinating area, with a couple of survivors which usually have a couple of interesting items. If you fancy an old-school pub in a lovely old Georgian building, go to the Jerusalem Cafe, on St John St (might be St John's lane), just off Clerkenwell Rd.